Spinal fixation system

ABSTRACT

A polyaxial pedicle screw includes a housing, a bone screw member, and an anvil. The bone screw member includes a head and a threaded shaft extending from the head. The head is selectively securable within the housing. The anvil is positionable within the housing adjacent to the head of the bone screw member when the anvil and the head of the bone screw member are positioned within the housing. The anvil may define one or more grooves in an outer surface of the anvil. The groove defines a flap that is flexibly attached to the anvil to enable the anvil to flex an amount sufficient to maintain the head of the bone screw in constant contact with the anvil when the bone screw member is moved relative to the anvil.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to, and the benefit of, U.S.Provisional Patent Application Ser. No. 61/375,354 filed Aug. 20, 2010,the entire contents of which are incorporated by reference herein.

BACKGROUND

1. Technical Field

The present disclosure relates generally to orthopedic surgery withparticular regard to spinal surgery. Specifically, the presentdisclosure relates to spinal fixation systems.

2. Description of Related Art

The spinal column is a complex system of bones and connective tissuesthat provide support for the human body and protection for the spinalcord and nerves. The adult spine is comprised of an upper and lowerportion. The upper portion contains 24 discrete bones, which aresubdivided into three areas including 7 cervical vertebrae, 12 thoracicvertebrae and 5 lumbar vertebrae. The lower portion is comprised of thesacral and coccygeal bones. The cylindrical shaped bones, calledvertebral bodies, progressively increase in size from the upper portiondownwards to the lower portion.

An intervertebral disc along with two posterior facet joints cushion anddampen the various translational and rotational forces exerted upon thespinal column. The intervertebral disc is a spacer located between twovertebral bodies. The facets provide stability to the posterior portionof adjacent vertebrae. The spinal cord is housed in the canal of thevertebral bodies. It is protected posteriorly by the lamina. The laminais a curved surface with three main protrusions. Two transverseprocesses extend laterally from the lamina, while the spinous processextends caudally and posteriorly. The vertebral bodies and lamina areconnected by a bone bridge called the pedicle.

The spine is a flexible structure capable of a large range of motion.There are various disorders, diseases and types of injury which restrictthe range of motion of the spine or interfere with important elements ofthe nervous system. The problems include, but are not limited to,scoliosis, kyphosis, excessive lordosis, spondylolisthesis, slipped orruptured discs, degenerative disc disease, vertebral body fracture, andtumors. Persons suffering from any of the above conditions typicallyexperience extreme or debilitating pain and often times diminished nervefunction. These conditions and their treatments can be furthercomplicated if the patient is suffering from osteoporosis, or bonetissue thinning and loss of bone density.

Spinal fixation apparatuses are widely employed in surgical processesfor correcting spinal injuries and diseases. When the disc hasdegenerated to the point of requiring removal, there are a variety ofinterbody implants that are utilized to take the place of the disc.These include, PEEK interbody spacers, metal cages and cadaver and humanbone implants. In order to facilitate stabilizing the spine and keepingthe interbody in position, other implants are commonly employed,including longitudinally linked rods secured to coupling elements, whichin turn are secured to the bone by spinal bone fixation fasteners suchas pedicle screws, hooks, and others. The opposing pair oflongitudinally linked rods is commonly disposed along the long axis ofthe spine via a posterior approach. Pedicle screws can be manufacturedfrom any biocompatible material, including cobalt chrome, stainlesssteel, titanium and PEEK (polyetheretherketone).

To meet the problem of providing a rigid pedicle screw and rodconstruct, especially for addressing the demands of stiff deformitycorrections, larger rod constructs have been made to improve thestrength of the screw and rod construct. Spinal rods are typically madeof titanium alloy. However, when large deformity corrections arenecessary these rods are not always strong enough. Larger diameterstainless steel rods have been made for these applications, but a largerrod requires a larger mating screw head to contain the rod, which, inturn, increases the profile of the construct. In addition, in order toreduce the likelihood of material incompatibility in vivo, the screwassembly also needs to be made of stainless steel to match the rodmaterial, which is not a cost effective alternative.

Therefore, a need exists for a cost effective, rigid screw and rodconstruct that can still maintain large, stiff deformity corrections.

SUMMARY

The present disclosure is directed to a polyaxial pedicle screwincluding a housing, a bone screw member, and an anvil. The bone screwmember is selectively positionable at a plurality of angles relative tothe housing when engaged to the housing and securable relative to thehousing at a cone angle of up to approximately 80 degrees. The bonescrew member includes a head and a threaded shaft extending from thehead. The head is selectively securable within the housing. The head mayinclude surface texture that frictionally engages with the anvil suchthat a user applied force is necessary to reposition the bone screwmember relative to the anvil when the bone screw member is disposed inengagement with the anvil. The head of the bone screw member includes afirst portion and a second portion. Only one of the first and secondportions enable the head to fit through an opening defined in thehousing and the other of the first and second portions maintains thehead of the bone screw member within the housing once inserted. One ofthe first and second portions may be cylindrically shaped and the otherof the first and second portions may be spherically shaped. Thecylindrically shaped portion enables the head to fit through the openingin the housing in an insertion orientation of the screw head relative tothe housing, and the spherically shaped portion maintains the head ofthe bone screw member within the housing once inserted and moved awayfrom the insertion orientation.

The anvil is positionable within the housing adjacent to the head of thebone screw member when the anvil and the head of the bone screw memberare positioned within the housing. A set screw is positionable withinthe housing to secure a rod member within the housing adjacent theanvil.

The anvil may define one or more grooves in an outer surface of theanvil. The groove defines one or more flaps. The one or more flaps areflexibly attached to the anvil to enable the anvil to flex an amountsufficient to maintain the head of the bone screw member in constantfrictional contact with the anvil when the bone screw member is movedrelative to the anvil.

The anvil may define a cavity having a surface with a plurality of radiiof curvature to accommodate rod members of variously sized diameters.The surface of the cavity may define a first section with a first radiusof curvature, a second section with a second radius of curvature, and athird section with a third radius of curvature. In this respect, theplurality of radii of curvature defines a compound curve that providestwo or more lines of contact on a plurality of different diameter rodmembers.

The outer surface of the anvil may have a non-round shape to preventdisorientation of the anvil when positioning the rod member adjacent theanvil. The anvil may include a protuberance on the outer surface of theanvil and the housing may define a slot on an inner surface of thehousing. The protuberance and the slot may be engagable to maintain thealignment of the anvil with respect to the housing.

A compression ring or cap may be securable to the housing to prevent thebone screw member from re-orienting to a position in which thecylindrically shaped portion of the screw head is aligned with thehousing opening. The housing may include a collar extending therefrom.The collar may define a cut out to facilitate the positioning of thehead within the housing. The compression ring or cap is securable (suchas by friction fit, gluing, welding or the like) to the collar to coverthe cut out after the head of the bone screw member is positioned withinthe housing.

According to one aspect, a polyaxial pedicle screw includes a bone screwmember, a housing, one or more wedge members, and one or more pins. Thehousing is positionable on the head of the bone screw member and definesone or more pin channels therethrough. One or more wedge members arepositionable within the housing adjacent the head to facilitate thesecurement of the head of the bone screw member and a rod member withinthe housing. A set screw is threadably engagable with an inner surfaceof the housing to secure a rod member adjacent the wedge member.

The one or more wedge members define one or more pin pockets in an outersurface thereof. The one or more pins are positionable within the one ormore pin channels. The one or more pin pockets maintain the one or morewedge members within the housing.

The bone screw member is selectively positionable at a plurality ofangles relative to the housing when engaged to the housing and securablerelative to the housing at a cone angle of up to approximately 80degrees. The bone screw member has a threaded shaft and a head. The headof the bone screw member includes a first portion and a second portion.Only one of the first and second portions enable the head to fit throughan opening defined in the housing and the other of the first and secondportions maintains the head of the bone screw member within the housingonce inserted. One of the first and second portions is cylindricallyshaped and the other of the first and second portions is sphericallyshaped. The cylindrically shaped portion enables the head to fit throughthe opening in the housing. The spherically shaped portion maintains thehead of the bone screw member within the housing once inserted.

The polyaxial pedicle screw may include a first wedge member and asecond wedge member. Each of the first and second wedge members arepositionable adjacent the head of the bone screw member. The first wedgemember and the second wedge member define a cavity having a surface witha plurality of radii of curvature to accommodate variously sized rodmembers. The surface of the cavity defines a first section with a firstradius of curvature, a second section with a second radius of curvature,and a third section with a third radius of curvature.

A compression ring or cap may be securable to the housing to prevent thebone screw member from re-orienting to a position in which thecylindrically shaped portion of the head of the bone screw member isaligned with the housing opening.

According to one aspect, a polyaxial pedicle screw includes a bone screwmember and a housing. The bone screw member includes a head having athreaded shaft extending from the head. The housing is positionable onthe head of the bone screw and has a distal opening. The bone screwmember is positionable at a plurality of angles relative to the housingwhen engaged to the housing and securable relative to the housing atangles within a cone angle of up to approximately 80 degrees. A setscrew is positionable within the housing to secure a rod member withinthe housing adjacent the anvil.

The head includes a first portion and a second portion. The first headportion is configured and dimensioned to pass through the housing distalopening and the second head portion is configured and dimensioned not topass through the housing distal opening. The first head portion mayinclude a substantially cylindrically shaped section. The cylindricallyshaped section defines a diameter that is less than the diameter of thehousing distal opening. The second portion of the head may define adiameter larger than the diameter of the housing distal opening. Thesecond portion of the head may include a substantially sphericalsection.

The housing may further define a collar extending from the distal end ofthe housing. The collar may define a cut out section. As can beappreciated, the screw assumes an insertion position with the neck ofthe screw disposed in the cut out section with the cylindrically shapedsection of the head aligned with the housing distal opening.

An anvil is positionable within the housing adjacent to the head of thebone screw member when the anvil and the head of the bone screw memberare positioned within the housing. The anvil defines one or more groovesin an outer surface of the anvil. The groove defines one or more flaps.The one or more flaps are flexibly attached to the anvil to enable theanvil to flex an amount sufficient to maintain the head of the bonescrew member in constant contact with the anvil. The anvil defines acavity having a surface with a plurality of radii of curvature toaccommodate variously sized rod members. The surface of the cavitydefines a first section with a first radius of curvature, a secondsection with a second radius of curvature, and a third section with athird radius of curvature such that the plurality of radii of curvaturedefines a compound curve that provides two or more lines of contact on aplurality of different diameter rod members. The outer surface of theanvil may have a non-round shape to prevent disorientation of the anvilwhen positioning a rod member adjacent the anvil. The anvil may includea protuberance on the outer surface of the anvil and the housing maydefine a slot on an inner surface of the housing. The protuberance andthe slot are engagable to maintain the alignment of the anvil withrespect to the housing.

The head of the bone screw member may include surface texture thatfrictionally engages with the anvil such that a user applied force isnecessary to reposition the bone screw member relative to the anvil whenthe bone screw member is disposed in engagement with the anvil without arod in the housing.

A cap is securable to the housing to prevent the bone screw member fromre-orienting to a position in which the screw head first portion isaligned with the housing distal opening.

According to yet another aspect, the present disclosure is directed to amethod of assembling a pedicle screw. The method includes providing apedicle screw including a bone screw member and a housing. The bonescrew member includes a head having a threaded shaft extending from thehead. The head includes a first portion and a second portion. Thehousing is positionable on the head of the bone screw and has a distalopening. The first head portion is configured and dimensioned to passthrough the housing distal opening and the second head portion isconfigured and dimensioned not to pass through the housing distalopening. The method involves the steps of positioning the screw relativeto the housing such that the first housing portion is aligned with thehousing distal opening, inserting the screw head through the housingdistal opening with the first head portion aligned with the distalopening, and rotating the screw relative to the housing such that thefirst head portion is no longer aligned with the housing distal openingand the second head portion prevents the screw head from exiting throughthe distal opening.

One step of the method includes mounting a collar onto the housing withthe screw head disposed in the housing to prevent the screw fromreassuming the position with the cylindrically shaped section alignedwith the housing distal opening.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects and features of the present disclosure willbecome more apparent in light of the following detailed description whentaken in conjunction with the accompanying drawings in which:

FIG. 1 is a perspective view of one embodiment of a polyaxial pediclescrew in accordance with the present disclosure;

FIG. 2 is an exploded, perspective view of the polyaxial pedicle screwshown in FIG. 1;

FIG. 3 is a side view of the polyaxial pedicle screw shown in FIG. 1;

FIG. 4 is a cross-sectional, side view of polyaxial pedicle screw shownin FIG. 1;

FIG. 5 is a bottom, perspective view of a housing of the polyaxialpedicle screw shown in FIG. 1;

FIG. 6 is a side, perspective view of an anvil of the polyaxial pediclescrew shown in FIG. 1;

FIG. 7 is a side cross-sectional view of the anvil shown in FIG. 6;

FIG. 8 is a perspective view of a set screw;

FIG. 9 is an exploded, perspective view of another embodiment of apolyaxial pedicle screw in accordance with the present disclosure;

FIG. 10 is a bottom, perspective view of a housing of the polyaxialpedicle screw shown in FIG. 9;

FIG. 11 is a perspective view of an anvil of the polyaxial pedicle screwshown in FIG. 9;

FIG. 12 is a side view of the anvil of FIG. 11 illustrating howvariously sized rod members are positioned relative to the anvil;

FIG. 13 is a perspective view of another embodiment of a polyaxialpedicle screw with a rod member secured thereto with a set screw inaccordance with the present disclosure;

FIG. 14 is an exploded, perspective view of yet another embodiment of apolyaxial pedicle screw in accordance with the present disclosure;

FIG. 15 is an enlarged, perspective view of an anvil of the polyaxialpedicle screw shown in FIG. 14;

FIGS. 16-17 are progressive perspective views illustrating assemblysteps of the polyaxial pedicle screw of FIG. 14 in accordance with thepresent disclosure;

FIG. 18 is an exploded, perspective view of yet another embodiment of apolyaxial pedicle screw in accordance with the present disclosure;

FIG. 19 is a perspective view of first and second wedge members of thepolyaxial pedicle screw shown in FIG. 18;

FIG. 20 is top view of the first and second wedge members shown in FIG.19; and

FIG. 21 is a cross-sectional view of the polyaxial pedicle screw in anassembled configuration.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Particular embodiments of the present disclosure will be describedherein with reference to the accompanying drawings. As shown in thedrawings and as described throughout the following description, and asis traditional when referring to relative positioning on an object, theterms “proximal” and “trailing” may be employed interchangeably, andshould be understood as referring to the portion of a structure that iscloser to a clinician during proper use. The terms “distal” and“leading” may also be employed interchangeably, and should be understoodas referring to the portion of a structure that is farther from theclinician during proper use. In addition, the term “cephalad” or“cranial” is used in this application to indicate a direction toward apatient's head, whereas the term “caudad” indicates a direction towardthe patient's feet. Further still, the term “medial” indicates adirection toward the middle of the body of the patient, whilst the term“lateral” indicates a direction toward a side of the body of the patient(i.e., away from the middle of the body of the patient). The term“posterior” indicates a direction toward the patient's back, and theterm “anterior” indicates a direction toward the patient's front. In thefollowing description, well-known functions or constructions are notdescribed in detail to avoid obscuring the present disclosure inunnecessary detail.

Referring initially to FIGS. 1-4, the present disclosure is directed toa polyaxial pedicle (multi-planar) screw that can provide a connection,via a spinal rod, between adjacent vertebrae not in the same plane. Oneembodiment of a polyaxial pedicle screw is generally referred to as 100.The polyaxial pedicle screw 100 includes a housing 110, a compressionring or cap 120, an anvil 130, a bone screw member 140, and a set screw150 (FIG. 8).

Referring to FIGS. 1-5, the housing 110 defines an opening 112therethrough that permits the reception of any suitable drivinginstrument (not shown) therethrough. The housing includes opposing walls110 a, 110 b that define a U-shaped channel 111 therebetween. Eachopposing wall 110 a, 110 b includes an external flanged recess 113 thatis configured to facilitate grasping of the housing 110 by an instrument(not shown) that can also be used to facilitate insertion of the pediclescrew 100 into a pedicle of a vertebral body. The internal surfaces ofopposing walls 110 a, 110 b include threaded portions 115 that arethreadably engagable with external threads 152 of the set screw 150(FIG. 8) to facilitate the securement of a rod member 160 (see FIGS. 12and 13) within the channel 111 of the housing 110 adjacent the anvil130.

As best shown in FIG. 2, the housing 110 includes a collar 114 extendingtherefrom. The collar 114 may have a smaller diameter than the diameterdefined by the opposing walls 110 a, 110 b of the housing 110. Thecollar 114 is adapted to facilitate the securement of the compressionring or cap 120 to the housing 110 once the bone screw member 140 issecured to the housing 110. The collar 114 has a cut out 114 a thatprovides a recess for the reception of a portion of the bone screwmember 140, namely a neck 146 of the bone screw member 140. The cut out114 a facilitates the positioning of the bone screw member 140 withinthe housing 110 from a distal end of the housing 110. In this respect,when the bone screw member 140 is positioned perpendicular (orsubstantially perpendicular; see e.g., FIG. 16) to the opening 112(e.g., positioned within a longitudinal axis “L” of the pedicle screw100 {see FIG. 1} and in axial alignment with a transverse axis “B”extending through cut out 114 a {see FIG. 5} such that a longitudinalaxis “C” of the bone screw member 140 {see FIG. 2} is perpendicular (orsubstantially perpendicular) to a longitudinal axis “A” of the housing110 which extends through opening 112), the neck 146 is positionedwithin the cut out 114 a and a head 142 of the bone screw member 140 ispositioned within the opening 112 with cylindrical surfaces (describedin greater detail below) of the bone screw head 142 also alignedperpendicular (or substantially perpendicular) to longitudinal axis “A.”To this end, upon rotation of a threaded shaft 144 of the bone screwmember 140 into coaxial alignment with longitudinal axis “A” of theopening 112 (see e.g., FIG. 17), the head 142, by virtue of a smallerdimensioned cylindrically shaped first portion 142 a of the head 142,slides into the housing 110. Once the screw head 142 is disposed insidethe housing 110 with the shaft 144 oriented away from the insertionposition, a larger dimensioned spherically shaped second portion 142 bof the head 142 maintains the bone screw member 140 within the housing110.

As discussed above, the bone screw member 140 includes a head 142 and athreaded shaft 144 extending from the head 142. The bone screw member140 may be a self-starting fastener or self-tapping fastener. Withreference to FIG. 2, the head 142 is selectively securable within thehousing 110 and includes a first portion 142 a and a second portion 142b. The head 142, as best illustrated in FIG. 4, includes a drivingrecess 142 c, which may be hexolobular or any other suitableconfiguration, defined in a proximal surface of the head 142. Thedriving recess 142 c is engagable with any suitable driving instrument(not shown) to enable the driving instrument to advance the bone screwmember 140 within bone. The first portion 142 a, which may havesubstantially cylindrical surfaces (but any suitable shape iscontemplated), enables the head 142 to fit through the opening 112defined in the housing 110 from the distal end of the housing 110. Inparticular, opposed cylindrical surfaces of the first portion 142 a maybe positioned in co-axial alignment transverse to axis “A”(simultaneously, the opposed spherical surfaces of the second portion142 b are coaxial with axis “A”; see FIG. 16) of the opening 112 (whilethe shaft 144 is perpendicular to axis “A” by virtue of the neck 146being seated in cut out 114 a) to enable the head 142 to securably fitinto housing 110 upon the rotation of the shaft 144 into coaxialalignment with axis “A.”. The second portion 142 b, which may havesubstantially spherical surfaces (but any suitable shape iscontemplated), maintains the head 142 of the bone screw member 140within the housing 110 once the head 142 is fully inserted from thedistal end of the housing 110 as discussed above.

The effective diameter of the first portion 142 a, e.g., the cylindricalsection of the screw head 142, is dimensioned to be smaller than thedistal opening 112 of the housing 110, so that the cylindrical sectioncan pass through the distal opening 112 into the housing 110. Thediameter of the second portion 142 b, e.g., the spherical section of thescrew head 142, is dimensioned to be larger than the diameter of thedistal opening 112 of the housing 110, so that when the screw head 142is disposed within the housing 110 and the cylindrical section is notaligned with the distal opening 112, the diameter of the sphericalsection prevents the screw head 142 from exiting the housing 110 throughthe distal opening 112.

As discussed below, the compression ring or cap 120 may be slid over theshaft 144 and affixed (such as by friction, threading, bayonette mount,gluing, ultrasonic or other welding or the like) to the collar 114 ofthe housing 110 to further secure the bone screw member 140 to thehousing 110. Once inserted, the bone screw member 140 is selectivelypositionable at plurality of angles relative to the housing 110 whenengaged to the housing 110. As illustrated in FIG. 1, the bone screwmember 140 may be fixedly securable relative to the housing 110 at acone angle α in the range of 60 to 80 degrees, preferably 70 degrees,from the longitudinal axis “L” extending through the polyaxial pediclescrew 100.

The compression ring or cap 120 may be securable to the collar 114 tocover the cut out 114 a after the head 142 of the bone screw member 140is positioned within the housing 110 to prevent the bone screw member140 from re-orienting to a position in which the first portion 142 a,namely the cylindrical surfaces, of the head 142 of the bone screwmember 140 is aligned with the opening 112 of the housing 110. As can beappreciated, when the bone screw member 140 is positioned within cut out114 a of collar as discussed above, the bone screw member 140 ispositioned transverse to the opening 112 and may be oriented with thecylindrical section aligned with the distal housing opening 112 suchthat the screw head 142 is movable into the housing 110 withoutinhibition from the second portion 142 b, which, as discussed above,otherwise maintains the bone screw member 140 within the housing 110once the screw head 142 is disposed inside the housing 110. Thus, thecompression ring or cap 120 covers the cut out 114 a portion of thecollar 114 so the shaft 144 of the bone screw member 140 cannot bere-oriented to a position in which the first portion 142 a, namely thecylindrical surfaces, of the head 142 are co-axial with the opening 112of the housing 110.

As depicted in FIG. 4, the anvil 130 is positionable within the housing110 adjacent the head 142 of the bone screw member 140 when the anvil130 and the head 142 of the bone screw member 140 are positioned withinthe housing 110 to facilitate the securement of a rod member 160 (seeFIGS. 12-13) within the housing 110. With brief reference to FIG. 8, theset screw 150 is positionable within the housing 110, e.g., viathreading engagement, to secure the rod member 160 within the housing110 adjacent the anvil 130. As can be appreciated, the set screw 150 maybe formed of titanium or titanium alloy. The set screw 150 includes adriving interface 154 that is engagable with any suitable drivinginstrument to threadably engage the set screw 150 within the housing110.

Referring to FIGS. 6-7, the anvil 130 includes a body 132 defining apassage 134 which permits the reception of a driving instrumenttherethrough. With particular reference to FIG. 7, the passage 134includes a top portion 134 a and a bottom portion 134 b separated by anedge 134 c. The bottom portion 134 b includes arcuate surfaces 135 toaccommodate spherical movement of the head 142 of the bone screw 140when positioned therein. The body 132 also defines an arcuate cavity 136at a proximal end thereof for the reception of a rod member 160 (FIG.12) which will be described in greater detail below. The body 132defines one or more grooves 138 a in an outer surface 132 a of the anvil130. Each groove 138 a defines one or more flaps 138 b. As depicted inFIG. 6, the anvil 130 may include a pair of opposing flaps 138 b thatact as cantilever beam springs. The one or more flaps 138 b are flexiblyattached to the anvil 130 adjacent the cavity 136 to enable the anvil130 to flex an amount sufficient to maintain the head 142 of the bonescrew member 140 in constant contact with the anvil 130 when the bonescrew member 140 is moved relative to the anvil 130. In this respect,the one or more flaps 138 b provide sufficient resistance (against innersurfaces of the housing 20) to prevent any laxity or unintended movementbetween the anvil 130 and the head 142 of the bone screw member 140. Ascan be appreciated, the spring force provided by the flaps 138 b createsfriction between the bone screw member 140 and the housing 110, thusstabilizing the assembly and making it easier to introduce a rod member160.

With continued reference to FIG. 6, the cavity 136 has a surface 137with a plurality of radii of curvature to accommodate variously sizedrod members 160. With brief reference to FIG. 12, a first rod member 160a having a first diameter is shown positioned in the cavity 136; incomparison, a second rod member 160 b having a second diameter is alsoshown positioned in the cavity 136. When first rod member 160 a isplaced against the arcuate surfaces of the anvil 130, the first rodmember 10 a nests easily against the anvil 130 because the first rodmember 160 a closely corresponds to the arc of the arcuate surfaces ofthe anvil 130. When the second rod member 160 b, a larger diameter rod,is positioned against the arcuate surfaces of the anvil 130 and the setscrew 150 is tightened against the second rod member 160 b, the secondrod member 160 b is similarly nested in the arcuate surfaces of theanvil 130 but is seated slightly more prominently. Of course, two roddiameters are shown in FIG. 12 for illustrative purposes only, and inpractice only one rod at a time could be placed in the housing.

Referring again to FIG. 6, the surface 137 of the cavity 136 defines afirst section 137 a with a first radius of curvature, a second section137 b with a second radius of curvature, and a third section 137 c witha third radius of curvature. In this respect, the plurality of radii ofcurvature defines a compound curve that provides two or more lines ofcontact on a plurality of different diameter rod members 160 (e.g., rodmembers 160 a, 160 b) without creating a stress riser in the anvil 130when any one of a plurality of different diameter rod members 160 ispositioned adjacent the compound curve.

The outer surface 132 a of the anvil 130 may have a non-round shape,e.g., slightly elliptical, to prevent disorientation of the anvil 130when positioning the rod member 160 adjacent the anvil 130.

As assembled, the bone screw member is fastenable to a bone structure(e.g. vertebra) and the housing 110 is repositionable in a plurality ofdirections with respect to the bone screw member 140 as discussed above.To this end, the housing 110 is rotatable about the longitudinal axis“L” (see FIGS. 1 and 3) extending through the polyaxial pedicle screw100 as well as pivotable relative to the longitudinal axis “L” and thebone screw member 140. A rod member 160, e.g., a spinal rod, ispositionable in the U-shaped channel 111 of the housing 110 and isnested against the arcuate surfaces of the anvil 130 as discussed above.The rod member 160 is then secured to the polyaxial pedicle screw 100using a set screw 150 (FIG. 8). To be more specific, the set screw 150is inserted into a proximal side of opening 112 of the housing 20adjacent the opposing walls 110 a, 110 b and rotated such that a distalend of the set screw 150 contacts the surface of the rod member 160 anddrives the rod member 160 and the anvil 130 towards the head 142 of thebone screw member 140. Once the desired angular position of the housing110 is reached, the set screw 150 is tightened further, which compressesthe rod member 160, the anvil 130, and the head 142 of the bone screwmember 140 within a recess 117 of the housing 110. The frictionalengagement between the head 142 of the bone screw member 140 and thebottom portion 134 b of the anvil 130 fixes the angular relationshipbetween the housing 110 and the bone screw member 140.

With reference to FIGS. 9-12, one embodiment of polyaxial pedicle screwis generally referred to as 200. Polyaxial pedicle screw 200 issubstantially similar to polyaxial pedicle screw 100 and is onlydescribed herein to the extent necessary to describe the differences inconstruction and operation thereof. Polyaxial pedicle screw 200 includesa housing 210, a bone screw member 140, an anvil 230, and a compressionring or cap 120. The housing 210 has one or more slots 212 defined on aninner surface 216 of the housing 210. The slots 212 may extendcontinuously and/or discontinuously along opposing walls 210 a and 210 band/or along a collar 214. The anvil 230 includes one or more flaps 236,one or more grooves 238, and one or more protuberances 234 on an outersurface 232 of the anvil 230. Like flaps 138 b, flaps 236 are movable inresponse to an applied pressure. The protuberances 234 and the slots 212are engagable to maintain the alignment of the anvil 230 with respect tothe housing 210 when the anvil 230 is positioned within the housing 210.One can appreciate that an aligned or substantially aligned arrangementof the anvil 230 with respect to the housing 210 maintains the U-shapedchannel 111 in a position to receive a rod member 160 that is alignedwith the polyaxial pedicle screw 200. As depicted in FIG. 11, the flaps236 and grooves 238 may be aligned or substantially aligned with theprotuberances 234.

With reference to FIG. 13, one embodiment of a polyaxial pedicle screw,generally referred to as polyaxial pedicle screw 300, is substantialsimilar to polyaxial pedicle screws 100, 200 and is only describedherein to the extent necessary to describe the differences inconstruction and operation thereof. Polyaxial pedicle screw 300 includesa housing 210, a bone screw member 340, an anvil 230 (see FIG. 11), anda compression ring or cap 120. As illustrated in this embodiment, bonescrew member 340 includes a head 142 (see FIG. 2) and a threaded shaft344. The threaded shaft 344 includes a first threaded segment 344 a anda second threaded segment 344 b.

With reference to FIGS. 14 and 15, one embodiment of a polyaxial pediclescrew, generally referred to as polyaxial pedicle screw 400, issubstantial similar to polyaxial pedicle screws 100, 200, 300 and isonly described herein to the extent necessary to describe thedifferences in construction and operation thereof. Polyaxial pediclescrew 400 includes a housing 210, a bone screw member 440, an anvil 430,and a compression ring or cap 120. Bone screw member 440 includes a head442 and a threaded shaft 444. The head 442 includes a first portion 442a and a second portion 442 b. The second portion 442 b includes surfacetexture 443, e.g., serrations, to facilitate the frictional engagementwith the anvil 430 and/or the housing 210 such that a user is requiredto apply some minimal force to move the bone screw member 440 relativeto the anvil 430 and/or the housing 210. In this respect, the bone screwmember 440 frictionally engages with the anvil 430 such that a userapplied force (e.g., beyond gravity forces) is necessary to repositionthe bone screw member 440 relative to the anvil 430 when the bone screwmember 440 is disposed in engagement with the anvil 430.

As best depicted in FIG. 15, the anvil 430 includes a body 432. The body432 includes a pair of opposing grooves 434, a pair of opposing flaps436, and a pair of opposing protuberances 438. The body 432 also definesa pair of notches 435 that extend along each protuberance 438. Theopposing flaps 436 each include a protuberance 436 a positioned betweena pair of notches 436 b. The protuberance 436 a and pair of notches 436b may be aligned or substantially aligned with protuberances 438 andpair of notches 435, respectively, as illustrated in FIG. 15.

With reference to FIGS. 18-21, one embodiment of polyaxial pedicle screwis generally referred to as 500. Polyaxial pedicle screw 500 issubstantially similar to polyaxial pedicle screws 100, 200, 300, 400 andis only described herein to the extent necessary to describe thedifferences in construction and operation thereof. Polyaxial pediclescrew 500 includes a housing 510, a compression ring or cap 520, ananvil 530, a bone screw member 540, and one or more pins 550. Thehousing 510 is positionable on a head 542 of the bone screw member 540.The housing 510 defines one or more pin channels 512 therethrough forthe reception of a pin 550 to secure at least a portion of the anvil 530within the housing 510.

As shown in FIGS. 19-21, the anvil 530 includes a first wedge member 532and a second wedge member 534. The first and second wedge members 532,534 are separate and distinct sections of the anvil 530, as most clearlyillustrated in FIG. 20. Each wedge member 532, 534 is positionablewithin the housing 510 adjacent the head 542 to facilitate thesecurement of the head 542 of the bone screw member 540 and a rod member160 (see FIG. 13) within the housing 510. Each wedge member 532, 534defines one or more pin pockets 536 in an outer surface 535 thereof. Inthis respect, a pin 550 is positionable within the one of the pinchannels 512 and one of the pin pockets 536 to maintain one of the firstand second wedge members 532, 534 within the housing 510, as bestillustrated in FIG. 21. As can be appreciated, any number of wedgemembers, pins, and/or pin channels may be utilized. In this embodiment,the head 542 of the bone screw member 540 need not be configured to haveboth cylindrical and spherical shaped sections, and the entire head 542can be substantially spherical. During assembly, each wedge 532, 534 isdisposed within the housing 510 in its proximal-most position, such thatthe spherical head 542 may enter a bottom opening 514 of the housing 510unimpeded. Once the head 542 is inserted, the wedges 532, 534 drop fromtheir proximal position to embrace the head 542 and prevent the head 542from being withdrawn from the housing 510.

A kit of the presently disclosed polyaxial pedicle screws may includetwo or more polyaxial pedicle screws of the present disclosure and oneor more rod members 160.

As can be appreciated, any portion of any of the presently disclosedpolyaxial pedicle screws can be formed of formed of titanium, titaniumalloy, stainless steel, cobalt chrome, or other metal or polymericmaterials. In this regard, it is also appreciated that utilizing acombination of compatible materials in the screw assembly may beadvantageous. Thus, it is contemplated that the housing could be made ofa harder or stiffer material such as cobalt chrome, while the screw andanvil and set screw may be made of another, compatible material such astitanium or titanium alloy. Further, components of any of the presentlydisclosed embodiments may be press fit, staked, pinned, or weldedtogether.

While several embodiments of the disclosure have been shown in thedrawings, it is not intended that the disclosure be limited thereto, asit is intended that the disclosure be as broad in scope as the art willallow and that the specification be read likewise. Therefore, the abovedescription should not be construed as limiting, but merely asexemplifications of particular embodiments. Those skilled in the artwill envision other modifications within the scope and spirit of theclaims appended hereto.

What is claimed is:
 1. A polyaxial pedicle screw, comprising: a housinghaving leading and trailing ends, and defining a longitudinal axis thatextends through the leading and trailing ends; a bone screw memberincluding a head and a threaded shaft extending from the head, the headbeing selectively securable within the housing; and an anvilpositionable within the housing adjacent to the head of the bone screwmember when the anvil and the head of the bone screw member arepositioned within the housing, the anvil defining at least one groove inan outer surface of the anvil, the at least one groove extendingtransverse to the longitudinal axis and defining at least one flap thatextends over the at least one groove, the at least one flap beingflexibly attached to the anvil to enable the anvil to flex an amountsufficient to maintain the head of the bone screw member in constantcontact with the anvil when the bone screw member is moved relative tothe anvil.
 2. The polyaxial pedicle screw of claim 1, wherein the anvildefines a cavity having a surface with a plurality of radii of curvatureto accommodate variously sized rod members.
 3. The polyaxial pediclescrew of claim 2, wherein the surface of the cavity defines a firstsection with a first radius of curvature, a second section with a secondradius of curvature, and a third section with a third radius ofcurvature such that the plurality of radii of curvature defines acompound curve that provides at least two lines of contact on aplurality of different diameter rod members.
 4. The polyaxial pediclescrew of claim 1, wherein the outer surface of the anvil has a non-roundshape to prevent disorientation of the anvil when positioning a rodmember adjacent the anvil.
 5. The polyaxial pedicle screw of claim 1,further comprising a set screw positionable within the housing to securea rod member within the housing adjacent the anvil.
 6. The polyaxialpedicle screw of claim 1, wherein the bone screw member is positionableat a plurality of angles relative to the housing when engaged to thehousing, the bone screw member being securable relative to the housingat a cone angle of up to approximately 80 degrees.
 7. The polyaxialpedicle screw of claim 6, wherein the head of the bone screw memberincludes a first portion and a second portion, wherein only one of thefirst and second portions enable the head to fit through a distalopening defined in the housing and the other of the first and secondportions maintains the head of the bone screw member within the housingonce inserted.
 8. The polyaxial pedicle screw of claim 7, wherein afirst one of the first and second portions includes a cylindricallyshaped section and a second one of the first and second portionsincludes a spherically shaped section, the cylindrically shaped sectionenabling the head to fit through the distal opening of the housing andthe spherically shaped section maintaining the head of the bone screwmember within the housing once inserted.
 9. The polyaxial pedicle screwof claim 8, further comprising a cap securable to the housing to preventthe bone screw member from re-orienting to a position in which thecylindrically shaped section of the head of the bone screw member isaligned with the distal opening of the housing.
 10. The polyaxialpedicle screw of claim 9, wherein the housing includes a collarextending therefrom, the collar defining a cut out to facilitatepositioning of the head within the housing, the cap being securable tothe collar to cover the cut out after the head of the bone screw memberis positioned within the housing.
 11. The polyaxial pedicle screw ofclaim 1, wherein the anvil includes a protuberance on the outer surfaceof the anvil and the housing defines a slot on an inner surface of thehousing, the protuberance and the slot being engagable to maintainalignment of the anvil with respect to the housing.
 12. The polyaxialpedicle screw of claim 1, wherein the head of the bone screw memberincludes surface texture that frictionally engages with the anvil suchthat a user applied force is necessary to reposition the bone screwmember relative to the anvil when the bone screw member is disposed inengagement with the anvil.
 13. The polyaxial pedicle screw of claim 1,wherein the at least one flap is spaced from the bone screw member. 14.A polyaxial pedicle screw comprising: a bone screw member including ahead, a neck, and a threaded shaft, the neck disposed between the headand the threaded shaft, the head including a first portion and a secondportion; and a housing positionable on the head of the bone screw andhaving a proximal end and a distal end, the distal end defining a distalopening and a cut out configured and dimensioned to receive the neck ofthe bone screw member to enable the head of the bone screw to beinserted into the housing through the distal opening, wherein the firstportion of the head is configured and dimensioned to enable the head topass through the distal opening of the housing and the second portion ofthe head is configured and dimensioned to prevent the head of the bonescrew from passing through the distal opening of the housing, whereinthe bone screw member is positionable between a first positionperpendicular to the housing and a second position in coaxial alignmentwith the housing, the neck receivable in the cut out with the bone screwmember in the first position while the head is positioned at leastpartially in the housing such that transitioning the bone screw memberto the second position, with the head remaining at least partiallypositioned in the housing, enables the head to be fully inserted withinthe housing.
 15. The polyaxial pedicle screw of claim 14, wherein thefirst portion of the head includes a substantially cylindrically shapedsection.
 16. The polyaxial screw of claim 15, wherein the cylindricallyshaped section defines a diameter that is less than a diameter of thedistal opening of the housing.
 17. The polyaxial screw of claim 14,wherein the housing further includes a collar that extends distally tothe distal end of the housing.
 18. The polyaxial screw of claim 14,wherein the second portion of the head defines a diameter larger than adiameter of the distal opening of the housing.
 19. The pedicle screw ofclaim 18, wherein the second portion of the head includes asubstantially spherical section.
 20. The pedicle screw of claim 14,further comprising an anvil positionable within the housing adjacent tothe head of the bone screw member when the anvil and the head of thebone screw member are positioned within the housing, the anvil definingat least one groove in an outer surface of the anvil, the at least onegroove defining at least one flap, the at least one flap being flexiblyattached to the anvil to enable the anvil to flex an amount sufficientto maintain the head of the bone screw member in constant contact withthe anvil.
 21. The polyaxial pedicle screw of claim 20, wherein theanvil defines a cavity having a surface with a plurality of radii ofcurvature to accommodate variously sized rod members.
 22. The polyaxialpedicle screw of claim 21, wherein the surface of the cavity defines afirst section with a first radius of curvature, a second section with asecond radius of curvature, and a third section with a third radius ofcurvature such that the plurality of radii of curvature defines acompound curve that provides at least two lines of contact on aplurality of different diameter rod members.
 23. The polyaxial pediclescrew of claim 20, wherein the outer surface of the anvil has anon-round shape to prevent disorientation of the anvil when positioninga rod member adjacent the anvil.
 24. The polyaxial pedicle screw ofclaim 20, further comprising a set screw positionable within the housingto secure a rod member within the housing adjacent the anvil.
 25. Thepolyaxial pedicle screw of claim 20, wherein the bone screw member ispositionable at a plurality of angles relative to the housing whenengaged to the housing and securable relative to the housing at angleswithin a cone angle of up to approximately 80 degrees.
 26. The polyaxialpedicle screw of claim 20, further comprising a cap securable to thehousing to prevent the bone screw member from re-orienting to a positionin which the first portion of the head is aligned with the distalopening of the housing.
 27. The polyaxial pedicle screw of claim 20,wherein the anvil includes a protuberance on the outer surface of theanvil and the housing defines a slot on an inner surface of the housing,the protuberance and the slot being engagable to maintain alignment ofthe anvil with respect to the housing.
 28. The polyaxial pedicle screwof claim 20, wherein the head of the bone screw member includes surfacetexture that frictionally engages with the anvil such that a userapplied force is necessary to reposition the bone screw member relativeto the anvil when the bone screw member is disposed in engagement withthe anvil without a rod member in the housing.